1. Field of the Invention
The invention relates to capacitors, particularly, to a electrochemical capacitor with carbon nanotubes.
2. Discussion of Related Art
Conventionally, capacitors used in electric circuits includes electrolytic capacitor, ceramic capacitors, and like. Capacitors provide energy storage as other energy storage devices, such as batteries or fuel cells. Capacitors store energy in the electric field between two oppositely charged parallel plates, which are separated by an insulator. The amount of energy a capacitor can store increases as the area of conducting plate increases, the distance between the plates decreases, and the dielectric constant of the insulating material increases. The electrolytic capacitor stores energy by charge separation across a thin insulating oxide film that is often formed by a controlled electrolytic oxidation process at an appropriate metal. Generally, a volumetric capacitance density of electrolytic capacitor is improved by using porous electrodes to create a large effective plate area.
Carbon nanotubes (CNTs) produced by means of arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). One of the characteristics of CNTs resides in that the aspect ratio of length to diameter is very large. In addition, the electric conductivity of carbon nanotubes is very high and allows a current flow at a current density of 100 MA/cm.sup.2 or more. Carbon nanotubes excel not only in electrical characteristics but also in mechanical characteristics. That is, the carbon nanotubes are distinctively tough, as attested by their Young's moduli exceeding 1 TPa, which belies their extreme lightness resulting from being formed solely of carbon atoms. Further, the carbon nanotubes have high elasticity, high resiliency and high chemical stability. These various and excellent characteristics tend to make CNTs ideal candidates for being used in field emission electronic devices, flat displays, as a hydrogen storage, and the like.
A new type of electrolytic capacitor using CNTs is disclosed. However, these CNTs are merely in contact with one another in a parent material. Therefore, it is difficult for the CNTs to provide stable electric conductivity and higher volumetric capacitance density.
What is needed, therefore, is a flexible electrolytic capacitor, which has stable and high electric conductivity, good charging/discharging property, and high volumetric capacitance density.